Radio communication systems in accordance with IEEE802. 11a/b/g have prevailed in recent years. Those systems are called “wireless LAN (local area network)” and do similar jobs wirelessly to what Ethernet (registered trademark) does, which is employed in a wire-LAN system.
The wireless LAN system has been employed in portable personal computers or wireless terminals at the beginning, and then prevails in a variety of fields. For instance, to eliminate the cumbersome work of wiring cables, use of the wireless LAN system is proposed for terminals that are bound to be used within a certain limited place.
One of the foregoing instances is this: the wireless LAN system is used for distributing content, e.g. video and audio, in aircraft. (Refer to Unexamined Japanese Patent Application Publication No. 2006-33564). The wireless LAN system installed in an aircraft transmits the content to wireless terminals available at each one of passengers' seats, so that the wireless LAN system can offer information distribution service to the passengers. The distribution system comprises the following structural elements:
a server for storing the content data to be offered to the passengers;
wireless access points (hereinafter referred to as access points), i.e. wireless base stations, connected to the server and placed within the aircraft in a given number; and
wireless terminals available at each one of passengers' seats.
Each one of the access points is located such that it can cover multiple wireless terminals.
FIGS. 6A and 6B are plan views of an aircraft fuselage for illustrating placement of access points of conventional radio communication systems 3 installed in the aircraft. FIG. 6A shows access point AP10 placed on the ceiling at the center of communicating zone 300 in the aircraft so that a single access point can cover all the wireless terminals mounted to every seat in the aircraft. FIG. 6B shows access points AP11, AP12, AP13, . . . , so that respective access points can cover each sub-zone produced by sub-dividing the communicating zone 300. The ovals drawn with alternate long and short dash lines in FIGS. 6A and 6B indicate arrival ranges of the radio waves transmitted from the respective access points.
FIG. 7 corresponds to FIG. 6B and shows a lateral view of the aircraft, and details the placement of access points. As shown in FIG. 7, communicating zone 300 is sub-divided into multiple sub-zones A1, A2, A3, . . . , and access points AP11, AP12, AP13, . . . , are positioned at approximate centers of respective sub-zones. Communication between the access points and each one of the passengers' seats thus can be provided.
In the foregoing case, to avoid interference between respective wireless terminals, each one of the access points is equipped with a directional antenna so that the radio wave can be transmitted to the terminals free from interference or disturbance with the other sub-zones, and a channel dedicated to the sub-zone is used between the access point and the wireless terminals within the sub-zone covered by the access point. On top of that, the channel has a different frequency from that of an adjacent sub-zone. The frequency bandwidth of the channel is time-divided, so that one access point can distribute information independently to each one of multiple wireless terminals.
Quality of the radio waves received by the wireless terminals has been improved by taking measures for a better location of the base station or better directivity of antennas installed at the receivers, thereby mitigating adverse effects of reflected waves to the communicating zone. (Refer to, e.g., Unexamined Japanese Patent Application Publication No. 2006-101106 and Unexamined Japanese Patent Application Publication No. H02-268528).
However, the access points set by the foregoing conventional methods have encountered the following problems: in the aircraft, wireless terminals are placed at the back of each seat, and the condition e.g. electric field strength, of radio waves received by the wireless terminals differs depending on the relative locations of the seats to the access point. The receipt qualities at the respective seats are thus unstable.
FIG. 8 shows wireless terminals 102 placed at the back of the respective seats, and each wireless terminal 102 includes an LCD and antenna 103, which is located not on the seat but on the rear of the backrest, because the design and function are taken into account.
In a case, where an access point AP is placed at the center of a communicating zone including seat-line SL in the interior space of the aircraft, the receiving environments of wireless terminals 102 differ depending on location F of access point AP (hereinafter referred to as AP location F) relative to longitudinal direction D of seat-line SL. Wireless terminals 102 placed ahead of AP location F receive direct waves W1 and W2 from access point AP, while wireless terminals 102 placed behind AP location F cannot receive the direct waves W1, W2 but receive indirect waves, i.e. transmitted waves W3, W4, and reflected wave W5.
The receipt condition of wireless terminals 102 thus differs greatly depending on the locations of terminals 102, i.e. ahead of AP location F or behind AP location F. In other words, the receipt condition greatly changes between a direct-wave area and an indirect-wave area. If the direct waves and the indirect waves are mixed together in the same communicating space, each one of the seats has a different receiving environment of the radio waves. In the case of aircraft, in particular, different from ordinary spaces such as an office space, the setting location is limited, and the devices of the system cannot be readily moved to get a better radio-wave environment. The radio communication system in the aircraft thus should find a uniform radio environment even a little better. If there is a need for changing antennas, an approval of the authorities is required for this change, so that it is hard to change antennas embedded at respective seats.
The problems discussed above need some measures to make the receipt quality of radio-wave uniform within the communication system; however, taking the measures invites the complexity of the system. Solving these problems is thus critical for operating the system.